1. Field of the Invention
This invention relates generally to the structure and processes for manufacturing a resistance temperature device (RTD). More particularly, this invention relates to an improved structure and manufacture process to provide a highly stable, reliable and low cost resistance temperature detector (RTD) by forming the resistance temperature detector with a surface mounted device (SMD) structure whereby a miniaturized RTD devices can be produced by mass production processes and the RTDs can be conveniently implemented in a wide variety of modern electronic applications.
2. Description of the Prior Art
Even though the temperature measurement conducted by employing a resistance temperature detector (RTD) is well known in the art, however, broad applications of the resistance temperature detector (RTD) are still limited by several technical difficulties. Specifically, the high precision low cost surface mount device (SMD) type packaging RTD is difficult to manufacture due to the incompatibility of the manufacturing processes and materials between which are commonly employed for RTD device and the SMD packaging technology. Additionally, connections to the input and output terminals of a RTD with conventional structure formed with the thin film resistors disposed on a flat substrate is also very inconvenient due to the requirements to form thin connection leads. A difficult task of managing many thin lead connections in very small space is commonly encountered. Labor intensive and time consuming lead welding or bonding processes are required to complete the task of lead connection. As more electronic devices are being miniaturized, the configuration of a conventional RTD often limits its usefulness due the difficulties in forming such small connections in very limited available spaces.
Conventional resistance temperature detectors (RTDs) are most commonly manufactured by either (a) forming wire coils around insulating material such as aluminum oxide or (b) applying a thin film process to form a layer of resistive RTD film on a substrate such as the aluminum oxide or silicon oxide substrate and then pattern the resistive film to the RTD. In the second method, in order to increase the adhesion of the RTD resistive film such as platinum film to the substrate, and to assure layer compatibility between the substrate and the RTD film, an intermediate layer, composed of a material such as inconel alloy or nickel to function as an interface between the substrate and the RTD film is often formed. After the RTD film is formed, the entire device supported on the substrate is annealed at an elevated temperature before the RTD resistor is patterned and trimmed to obtain the designed resistance value. A dicing process is then carried out to divide the RTDs formed on the substrate into chips. These chips are further processed by parallel gap welding, and high temperature passivation layer formation. Each chip is then packaged as a RTD device. The processes of welding and packaging involve procedures which are time consuming and cannot be conveniently automated. Due to these limitations, the production costs of RTD devices are high. Additionally, in order to apply these RTD devices to temperature measurement, the RTD often has to be welded on for connection to terminals to supply a RTD voltage. The costs and complexity of applying the RTDs are further adversely affected by these connection welding requirements. For these reasons, the RTDs are typically employment only for industrial applications or high temperature measurements. Due to its high production costs, the RTDs are seldom employed in measurement of temperature of lower ranges. Moreover, the RTDs are not suitable for applications where limited space are available due to the requirements of welding onto the power supply terminals by the use of electrical wires for providing power source to the thin film resistors for measuring the temperature.
Many prior art discloses various types of RTDs wherein these RTDs are basically provided with a similar structure which requires lead connectors and welding of these lead connectors to other existing circuits for providing voltage source to the RTDs. One example of this is a platinum resistance thermometer. Jinda et al. disclose in U.S. Pat. No. 4,805,296 entitled "Method of Manufacturing Platinum Resistance Thermometer" (issued on Feb. 21, 1989) a method of manufacturing a resistance thermometer by preparing a support substrate and forming a platinum film serving as a temperature measuring element. The platinum is formed by a sputtering process on the substrate with an aluminum oxide film serving as a stabilizing layer to improve the stability and reproducibility of the sensor characteristics, namely the platinum layer. FIGS. 1A and 1B are respectively a cross sectional view and a perspective view of this RTD device where lead wires 5 are employed for connection to a voltage source.
Toenshoff discloses in U.S. Pat. No. 4,146,957, entitled "Thick Film Resistance Thermometer" (issued on Aug. 3, 1970), a folded, highly pure platinum, thick film path on a ceramic cylinder substrate for providing a high temperature coefficient of resistance thermometer. A rotary thick film printing technique is used to form the platinum resistor path on the ceramic cylinder. As shown in FIG. 2, again, lead wires 14 are employed for external connections. For modern electronic applications, the task of welding these wire leads to the circuits usually disposed on a printed circuit boards is labor intensive and time consuming. In addition, for electronic device where miniaturization is a primary consideration, the use of RTD with wire lead connection can be very inconvenient due to the need of welding the leads to circuits with very limited working space and typically provided points of connection of very small dimensions.
Therefore, a need still exists in the art of design and structure of linear thermometers and the manufacture processes and package of RTDs to provide an improved structural configuration and manufacture method for packaging of the linear thermometers which is simpler in structure, easier for streamlining and automating the manufacture processes while providing better interface connection features such that the liner thermometers such as RTDs can be manufactured at lower costs and be conveniently implemented in broad variety of temperature measurement applications.